Just pass the parameter action
with the action name.
response = client.post(change_url, {'action': 'mark_as_read', ...})
Checked items are passed as _selected_action
parameter. So code will be like this:
fixtures = [MyModel.objects.create(read=False),
MyModel.objects.create(read=True)]
should_be_untouched = MyModel.objects.create(read=False)
#note the unicode() call below
data = {'action': 'mark_as_read',
'_selected_action': [unicode(f.pk) for f in fixtures]}
response = client.post(change_url, data)